Voice VLAN Access/Abuse Possible on Cisco voice-enabled, 802.1x-secured Interfaces

[csirt () fishnetsecurity com]

==========================================================================
Title: Voice VLAN Access/Abuse Possible on Cisco voice-enabled, 802.1x-secured Interfaces Vulnerability Discovery: 
FishNet Security - http://www.fishnetsecurity.com
Date: 06/08/2005
Severity: Medium - Voice VLAN locally accessible despite voice-enabled ports being 802.1x-secured
Vendor: http://www.cisco.com
==========================================================================
==========================================================================

Summary:

Cisco switches that support both 802.1x security and Cisco IP Phones have the ability to differentiate between access 
of the voice VLAN by Cisco IP Phones and access of the data VLAN by devices connected to the auxiliary ports 
(daisy-chained) of IP Phones. Thus 802.1x port-level security can be achieved on switch ports connected to Cisco IP 
Phones which are, in turn, connected to end-user devices.

--------------------------------------------------------------------------

Description of Issue:

In this configuration data VLAN access provided to devices connected to IP Phone auxiliary ports is authenticated via 
802.1x. Unfortunately access to the voice VLAN cannot be so securely authenticated due to the lack of 802.1x supplicant 
software in Cisco IP Phones. It has been found that a specifically crafted Cisco Discovery Protocol (CDP) message is 
sent from the Cisco IP Phone to the switch which opens access to the voice VLAN for frames originating from that Cisco 
IP Phone's MAC address. Although 802.1x port-security may be configured on the switch port voice VLAN access is 
trivially gained by spoofing a CDP message.

--------------------------------------------------------------------------

Risk Mitigation:

There is no *fix* to this issue as of yet. The true resolution would be to provide 802.1x supplicant software on IP 
phones such that voice VLAN and data VLAN access are both 802.1x authenticated. Traditionally, access to the voice VLAN 
of a voice-enabled system such as is described above was provided by a switch to any device without authentication. 
Cisco has provided the ability to differentiate between phones and other devices albeit in a such away that voice VLAN 
access is still trivially gained. It should be noted that this configuration is still preferred over the old method 
which uses no  authentication for either VLAN. However, it is still important to note that true port-level 
authentication is still not being provided. Currently the best way to mitigate the risk introduced by unauthorized 
voice VLAN access is to implement traditional security measures as well as some of the advanced security features 
available in Cisco networking equipment. Cisco CallManager 4.x and 
 certain Cisco IP Phones now support the authentication of phone registration through the use of certificates. Features 
like this reduce the risk of unauthorized voice VLAN access if other necessary controls are also put into place such as 
the following: 

* Disable telnet on phones.

* Always use cryptographically secure management protocols such as SSH, HTTPS, and SNMPv3 when possible to lower the 
risk of eavesdropping that ARP poisoning and DNS manipulation attacks present.

* Disable all administrative access to network infrastructure from voice VLAN addresses.

* Configure dynamic ARP inspection to lower the risk of ARP poisoning attacks.

* Configure DHCP snooping to lower the risk of DHCP server spoofing attacks.

* Configure limits on the amount of MAC addresses allowed to be connected to a switch port. This will lower the risk of 
port-stealing by overwhelming the switch CAM table.

* Configure storm control to limit the risk of a DOS attack via non-unicast traffic.

* Configure proper filtering between voice and data networks to ensure that even if unauthorized voice VLAN access is 
achieved the risk presented by this access is less than the risk posed by unauthorized data VLAN access.

--------------------------------------------------------------------------

References:
http://www.fishnetsecurity.com/csirt/disclosure/cisco/
http://www.cisco.com/en/US/netsol/ns340/ns394/ns171/ns128/networking_solutions_white_paper09186a00801b7a50.shtml